In recent years, a self-luminous display device having a pixel formed using a light emitting element such as an electroluminescent (EL) element, i.e., a light emitting device has attracted attention. As a light emitting element used for such a self-luminous display device, an organic light emitting diode (OLED) and an EL element have attracted attention, which have been used for an EL display or the like. Since these light emitting elements emit light by themselves, they have advantages over a liquid crystal display in higher pixel visibility, no backlight required, and higher response speed. Note that the luminance of most of light emitting elements is controlled by a current value flowing to the light emitting element.
In addition, development of an active matrix display device has been advanced, in which each pixel is provided with a transistor for controlling light emission of a light emitting element. The active matrix display device is expected to be put into practical use because not only can it achieve high-definition and large-screen display that is difficult for a passive matrix display device, but also it operates with less power consumption than a passive matrix display device.
A pixel configuration of a conventional active matrix display device is shown in FIG. 45 (Reference 1: Japanese Published Patent Application No. H8-234683). The pixel shown in FIG. 45 includes thin film transistors (TFTs) 11 and 12, a capacitor 13, and a light emitting element 14, and is connected to a signal line 15 and a scan line 16. Note that either a source electrode or a drain electrode of the TFT 12 and one electrode of the capacitor 13 are supplied with a power supply potential Vdd, and an opposite electrode of the light emitting element 14 is supplied with a ground potential.
When amorphous silicon is used for a semiconductor layer of the TFT 12 which controls a current value supplied to the light emitting element, that is, a drive TFT, fluctuations of the threshold voltage (Vth) occur due to deterioration or the like. In that case, although the same potential is applied to different pixels through the signal line 15, a current flowing to the light emitting element 14 differs from pixel to pixel and the resulting luminance becomes nonuniform among pixels. Note that in the case of using polysilicon for the semiconductor layer of the drive TFT, characteristics of the transistor deteriorate or vary likewise.
In order to overcome the above problem, an operating method using a pixel in FIG. 46 is proposed in Reference 2 (Reference 2: Japanese Published Patent Application No. 2004-295131). The pixel shown in FIG. 46 includes a transistor 21, a drive transistor 22 which controls a current value supplied to a light emitting element 24, a capacitor 23, and the light emitting element 24, and the pixel is connected to a signal line 25 and a scan line 26. Note that the drive transistor 22 is an NMOS transistor. Either a source electrode or a drain electrode of the drive transistor 22 is supplied with a ground potential, and an opposite electrode of the light emitting element 24 is supplied with Vca.
FIG. 47 shows a timing chart of the operation of this pixel. In FIG. 47, one frame period is divided into an initialization period 31, a threshold (Vth) write period 32, a data write period 33, and a light emitting period 34. Note that one frame period corresponds to a period for displaying an image for one screen, and the initialization period, the threshold (Vth) write period, and the data write period are collectively referred to as an address period.
First, in the threshold write period 32, the threshold voltage of the drive transistor 22 is written into the capacitor 23. After that, in the data write period 33, a data voltage (Vdata) indicative of the luminance of the pixel is written into the capacitor 23, and thus Vdata+Vth is accumulated in the capacitor 23. Then, in the light emitting period 34, the drive transistor 22 is turned on, so that the light emitting element 24 emits light at a luminance specified by the data voltage by changing Vca. Such operation can reduce luminance variations caused by fluctuations of the threshold voltage of the drive transistor.
Reference 3 also discloses that a gate-source voltage of a drive TFT is set at a voltage corresponding to the sum of a data potential and the threshold voltage of the drive TFT, and thus a current flowing to a light-emitting element does not change even when the threshold voltage of the TFT fluctuates (Reference 3: Japanese Published Patent Application No. 2004-280059).
In each of the operating methods disclosed in References 2 and 3, the initialization, the threshold voltage writing, and the light emission are performed by changing a potential of Vca several times in each frame period. In these pixels, one electrode of a light emitting element which is supplied with a potential Vca, that is, an opposite electrode is formed entirely over the pixel region. Therefore, the light emitting element cannot emit light if there is even a single pixel in which data writing operation is performed besides initialization and threshold voltage writing. Thus, the ratio of a light emitting period to one frame period (i.e., duty ratio) becomes low as shown in FIG. 48.
When the duty ratio is low, the amount of current supplied to a light-emitting element through a driving transistor has to be increased; therefore, a voltage applied to the light-emitting element becomes higher, which results in high power consumption. Further, since the light-emitting element and the driving transistor will easily degrade with a low duty ratio, even higher power is required for obtaining about the same level of luminance as that before degradation.
In addition, since the opposite electrode is connected to all of the pixels, the light-emitting element functions as an element with large capacitance. Accordingly, in order to change the potential of the opposite electrode, high power consumption is required.